Energy Fact Sheet No: 12 – August 2016
Straw for Energy
Burning straw to generate electricity or
heat may seem like sacrilege to livestock
farmers but it’s an expanding industry
in the tillage sector across Europe and
it provides growers with an alternative
income stream.
Straw is a by-product resulting from the
growing of commercial crops, primarily
cereal grain. Ireland’s area under cereals
amounts to almost 300,000 hectares and
yielding approximately 1.1m tonnes of
straw. The traditional markets for straw
include, animal bedding, animal feed and
chopping and plough back to increase
soil carbon content. Straw has been
used in other EU countries for decades
as a combustion fuel for both heat and
electricity production.
The Irish government and the EU have
made clear their wish to promote the use
of increasing amounts of biomass for the
production of energy. They have set overall
targets for renewable energy production
(16% of total final consumption from
renewables by 2020), and a specific target
for renewable heat (12% substitution) in
the National Bioenergy Action Plan. Both
targets are far in excess of what is being
produced from biomass at present.
Cereal straw is a biomass material that
is in plentiful supply in Ireland. An SEIcommissioned study in 2005 estimated
that of a total straw production of 1.2 - 1.5
million tonnes, 80-320,000 tonnes could
be available for energy use.
Traditionally straw prices have been very
volatile, but fixed price contracts are likely
to be more attractive to growers now than
they have been in the past.
STRAW FOR ENERGY
boiler warranty to straw. Straw is a CO2 neutral fuel and that
is the reason why it should be promoted in the energy supply
chain. Straw has been used as an energy fuel in Denmark since
the early seventies.
For the use of straw as fuel, its use in boilers might possibly
lead to corrosion problems on heat exchange surfaces. There
are a number of boiler manufacturers who will apply their
Table 1 – Average Straw Yield
Type Hectares
t/straw/*ha (DM)
t/grain*ha (DM)
t/straw
total
W. Wheat
S. Wheat W. Barley S.Barley W.Oats S.Oats 4.2 (3.6) 3.0 (2.5) 4.2 (3.6) 3.6 (3.0) 4.7 (4.0) 3.9 (3.3) 9.6 (7.89) 6.4 (5.28) 8.3 (6.8) 6.5 (5.36)
7.8 (6.4) 7.8 (6.4) 401,380
160.740
80,180
364,800
72,096
13,065
85,400 42,300 21,100 114,000 18,024 4,083 *Straw at 15% moisture and grain at 18% moisture
Table 2 – Energy Value of Straw
Type
Calorific
Energy
Value
Content
(Mj/Kg)
(kWh)
Wheat Straw
14.4
4,032
Barley Straw 14.7 4,116 Rape Straw 14.3 4,004 Meadow Hay 14.3 4,004 Source, Teagasc figures
Heating
Ash
Oil
content
Equivalent
(litres~)
396
57
406 48
393 62
393 71
Straw for Energy
STRAW HARVESTING
Winter crops of Barley, Wheat, Oats and Oilseed Rape
would become available from July – August. The spring
sown crops would become available in late August to end of
September. Straw is normally baled in round or square bales
and transported from the field to storage or directly off-farm.
Straw may also be recycled to the soil to improve soil structure
and increase soil organic matter content.
Table 3 – Approximate Bale Weights
Small square bale
4 x 4 round bale 5 x 4 round bale 8 x 3 x 2 (square) 8 x 3 x 3 (square) COMPETING USES
15kg
150 kg
240 kg
150 kg
450 kg
8 x 4 x 4 (square) 625 kgStraw used for fuel purposes usually
contains 14 – 20% moisture that vaporises during burning.
The remaining dry matter consists of less than 50% carbon,
6% hydrogen, 42% oxygen, and small amounts of nitrogen,
sulphur, silicon and other minerals e.g. alkali (sodium and
potassium) and chloride.
STRAW USE
Straw can enter any of the following markets:
=Animal feed (Barley)
= Mushroom compost (Wheat)
= Animal bedding (Oaten, Barley, wheaten)
= Chopping to return to soil (mainly on winter crops)
OILSEED RAPE STRAW
Oilseed rape straw removes fewer nutrients than cereals and
so is less ‘valuable’. However, it has a high calorific value and
burns very well.
Straw
Barley
Triticale
Rape
Wheat
C
47.5
43.9
47.1
45.6
H
5.8
5.9
5.9
5.8
O
41.4
43.8
40.0
42.4
N
0.46
0.42
0.84
0.48
Assessment of the Renewable Energy Resource Potential of
Dry Agricultural Residues in Ireland’ (RPS SEI, 2004), it was
concluded that by applying the established rule of thumb
of straw being approximately two thirds of the crop yield is
a reasonable estimate. Therefore based on a yield of 2.1MT
per annum, the annual average theoretical straw resource is
therefore estimated as being 1.4MT. Applying a percentage
variance of +/-10% (percentage variance of annual grain
yields; 1985-2013F), the theoretical straw resource in Ireland
is estimated in the range 1.26MT to 1.55MT.
K
1.38
1.05
0.79
1.01
STRAW RESOURCE
As a general rule there is a ratio between grain and straw/chaff
somewhere between 55:45 and 50:50 and a proportion of
the cereal crop is always left in the field following harvesting
as stubble. However, there does not appear to be a precise
relationship between grain yield and straw yield. The average
grain (oats, wheat and barley) yield in Ireland over the period
1985 to 2013F is calculated as being approximately 2.01 MT
tonnes. It should be noted that this long-term average includes
grain yields for years 1993 (1.63MT) and 1994 (1.61MT),
which were low due to EU policy reforms and exposure to
world market prices. The average grain yield over the period
2008-2011 was 2.3MT.
In the 2004 SEAI commissioned report entitled ‘An
The SEAI figures collated in 2003 suggests that roughly 1MT
tonnes of straw (70% of theoretical yield) per annum are used
on-farm as bedding and feeding. Given the improvements in
farm waste management practices and the increase in overall
annual straw yields, it is considered reasonable that this
percentage has decreased to around 65% of the current total
straw resource.
The next largest user of straw in Ireland is the mushroom
composting industry which uses wheaten straw to produce new
mushroom compost. Mushroom compost is manufactured
from wheaten straw and poultry manure with the addition of
water and gypsum (calcium sulphate).
Based on the mushroom composition percentages and the
amount of compost used in 2011, it is estimated that the
mushroom compost industry uses around 85,000 tonnes of
wheaten straw produced in Ireland annually (5.5 % of the
total theoretical straw resource).
Given that the present-day combined estimate of straw used
for animal husbandry and mushroom composting is around
71% of the theoretical resource of 1.4MT, it is considered
reasonable that 25-30% of the theoretical resource (350,000
- 420,000 tonnes) would be available for energy purposes.
Ca
0.49
0.31
1.70
0.31
Mg
0.07
0.05
0.22
0.10
P
0.21
0.08
0.13
0.10
S
0.089
0.056
0.270
0.082
CI
0.40
0.27
0.47
0.19
BOILERS
There are a number of boilers developed which can handle
the more complex chemistry of straw for combustion. Batch
– fired boilers are available equipped with combustion air fans
to control the air supply. Boiler plants for straw can be used for
on-farm heat generation and for district heating. Straw can be
presented in chaffed, pellet or whole bale form depending on
the boiler technology.
Prepared by:
Barry Caslin, Teagasc Rural Economy Development Programme
Email: [email protected] Tel: 076/1111213
Dr John Finnan Teagasc Oak Park, Carlow
Email: [email protected] Tel: 059/9170253
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Naas Printing Limited